Does anyone have a nicely packaged way to do this? I don't see any
nice boundaries or variables in Makeconfig that one could take
advantage of.

IIRC there is supposed to be a target to run a single test, and this
target might know enough to be a good starting point?
If you are going to add some debugging functionality, I would like to
offer at no cost my feature wishlist.
1. Build normal dynamic executable test (Easy)
2. Build only build tree dynamic linker set test.
3. Build only DT_RPATH set to build tree directories test.
4. Build with both 2 and 3.
5. Build static version of the test (watch out for the right *.a files!)
6. Build a version of the test that can be turned into preprocessed
source for creating reduced testcases (this involves somehow removing
the use of -include libc-symbols.h).
Go get em! :-)
Cheers,
Carlos.